This invention relates to a method of prestressing and reinforcing damaged pipes and other cylindrical structures. In particular, this invention relates to lowering stress in the damaged pipe by compressing the pipe with a prestressing resin injected between the pipe and a strengthening wrap, thereby effecting a repair and allowing the pipe to operate at design fluid pressure, as well as design flow levels.
Pipe systems provide basic utilities to the public, such as water, gas and sewage. These basic utilities are often heavily relied upon by the public. As such, the pipe systems are usually in a state of full operation, i.e. optimal flow rate. There is usually a very limited amount of time that the pipes and pipe systems operate at a reduced flow rate, while not seriously affecting utility service.
Pipelines and other cylindrical structures, whether above ground or below, occasionally suffer damage. Over time, these cylindrical structures can be damaged from rust, corrosion or other degradation. Further, the damage can occur during repair or installation, for example from tool impact or falling debris.
At a damaged area of the pipe, there is a smaller cross-sectional thickness of the pipe. The thickness of the pipe is inversely proportional to the hoop stress of the pipe, that is, the stress due to the operational fluid pressures in the pipe. Accordingly, there is a larger hoop stress in the damaged area as compared to the rest of the pipe.
The hoop stress in the pipe is proportional to the strain in the pipe. Therefore, the larger hoop stress in the damaged area results in a proportionally larger strain. The larger strain in the damaged area of the pipe causes further damage. In order to prevent further breakage or leaking from the structure, the flow through the pipe, and resulting pressure therefrom, is typically reduced or stopped. However, this flow reduction is undesirable because the utility service is correspondingly reduced. Accordingly, a repair method that results in no interruptions of utility service is desired.
It is usually more cost effective to permanently repair the leaking pipe rather than replace it. When the damaged pipe is replaced, the downtime of the system, the labor costs, the material costs, as well as the costs associated with the loss of utility services are usually large.
Cylindrical structures that may need repair may comprise metal, concrete, composites, fiberglass, and plastics, including polyethylene, PVC, polyurethane. However, there is no universal method of pipe repair for these various materials. Composite materials are used to repair steel, copper, and PVC pipes, but there is no current solution for the repair of polyethylene type pipes. This is because traditional epoxy and phenolic resin systems do not adequately adhere to the polyethylene type pipes. Accordingly, a universal method of pipe repair for various materials is desired.
A pressurized vessel or pipe is operated in a pressurized fluid system. A fluid in the pressurized vessel is intended to be present at a predetermined pressure. The vessel has a wall that has a condition in a localized area that creates a hoop stress that is greater than the pipe""s design hoop stress at the design fluid pressure. To prevent further damage to the pipe, a compressive stress is created in the localized area of the vessel wall. The compressive stress is correlated in magnitude to the design amount of tensile stress so that, when the compressive stress is arithmetically combined with the actual tensile stress in the localized area, the total stress is at or near the design amount of tensile stress. As the pipe is near the design level of tensile stress, the fluid can then be present in the vessel at the predetermined design fluid pressure.
The compressive stress in the pipe is accomplished by tightly wrapping a strip of flexible substrate around the localized area of the pipe. The substrate is impregnated with a resin that cures upon contact with an activator, such as moisture, air, water, or an organic solvent or agent. In addition, a prestressing resin is injected under pressure between the substrate and the pipe in the damaged area. If the resin is directly injected between the substrate and the pipe, a mid-section of the substrate that faces the damaged pipe area is provided with tape or the like. The tape is fastened to the inside surface of the substrate so that the substrate does not adhere directly to the pipe, and can be expanded and spaced from the pipe upon the resin injection. Alternatively, a prestressing resin is injected under pressure into a bladder inserted between the substrate and the pipe. The tight wrapping of the substrate around the pipe, in combination with the prestressing resin, subjects the pipe to the desired compression forces, i.e. a negative hoop stress.